This invention relates to an electrochromic display device which utilizes a combination of two types of electrochromic materials one of which takes on color in its electrochemically oxidized state while the other takes on color in its electrochemically reduced state. The display device includes an auxiliary electrode for use in initially reducing one of the two electrochromic materials.
It is known to use a combination of the aforementioned two types of electrochromic materials in an electrochromic display device having oppositely arranged two electrode layers. More particularly, each electrode layer in such a device is comprised of a transparent electrode film deposited on a transparent substrate and a coating layer of an electrochromic material, and the two types of electrochromic materials are assigned to the two electrode layers, respectively. For example, Prussian blue which assumes blue color in its electrochemically oxidized state and becomes colorless by reduction and tungsten trioxide WO.sub.3 which is colorless in its electrochemically oxidized state and assumes blue color in a reduced state are used in combination. In operation of the display device, electrochemical oxidation of the electrochromic material of one electrode is accompanied by electrochemical reduction of the opposite electrode. Accordingly simultaneous coloration of the two electrodes and simultaneous bleaching of the two electrodes take place. The primary purpose of this construction is intensifying coloration of the diplay device.
However, it is inevitable that both of the two kinds of electrochromic coating layers as formed are in electrochemically oxidized state, so that one of the two electrodes assumes color whereas the other is colorless. Therefore, it is necessary to accomplish electrochemical reduction of one of the two electrochromic coating layers precedent to actual operation of the electrochromic display device. For use in the initial reduction treatment an auxuliary electrode is disposed in a marginal region of the electrochromic display device, and it is known to use an electrochemically and reversibly oxidizable and reducible substance as the principal material of the auxiliary electrode as shown, for example, in JP-A No. 59-159134 and Japanese utility model application No. 60-129565.
The aforementioned auxiliary electrode is usually provided with an insulating covering the material of which is permeable to ions, so that ions can migrate from the electrolyte confined in the display device to the auxiliary electrode, and vice versa. Such a manner of insulation of the auxiliary electrode offers little problem if the distance between the two substrates of the electrochromic display device is large enough to disposition of the auxiliary electrode without contacting with the transparent electrode films deposited on the substrates or the electrochromic layers formed thereon. However, in recent electrochromic display devices the distance between the two substrates is seldom so large since it is desired to reduce both the total thickness and gross weight of the display device. Then it is almost inevitable that the insulating covering of the auxiliary electrode comes into tight contact with the transparent electrode films or the electrochromic coating layers, and therefore a leakage current flows between the auxiliary electrode and the electrochromic layers. Such a leakage current raises the need of extending the duration of the initial reduction treatment and, besides, in actual operation of the electrochromic display device becomes a cause of a relatively short memory time of the display device, i.e. insufficient capability of maintaining the colored or bleached state after termination of application of the coloration or bleaching voltage.